Having the WWW being Regina’s sister just feels so ret-conny to me. Cora basically staked all her hopes and dreams of ruling on to Regina. To reveal that she has another child just feels so outta place. And I don’t think she had a baby before Henry Sr.
I could easily see Cora having had a baby out of wedlock and trying to hush up that fact because she said royal brides were expected to be snow white. Something tells me Cora was anything but virginal, since I’m pretty sure she and Rumple were lovers, but she would want to give the impression of being unsullied when she married Prince Henry. If so, then if she had another child by someone else, she might’ve made certain no one ever discovered she had an older daughter, the WWW. That is why she put all of her hopes into raising Regina, her legitimate child of noble blood, and why Cora might’ve despised her eldest daughter and why she ended up being raised in another world entirely, Oz.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy